(1) Conventional thin ferroelectric film capacitors have a laminate structure consisting of a Pt upper electrode/ferro electric layer (PZT)/Pt lower electrode as described, for example, in “Thin Ferroelectric Film Memory” (published by Science Forum (1995), p 227).
(2) Further, in “Method of Forming a Thin Film” described, for example, in Japanese Patent Unexamined Publication Hei 7-142600, a compound BaTiO3 is formed on a thin Pt film, while the crystal orientation of the thin Pt film is reflected therein as it is, thereby making it possible to easily control the orientation property of the thin ferroelectric film and control the degree of residual dielectric polarization.
(3) Further, in “Method of Manufacturing a Thin ferroelectric film” described, for example, in Japanese Patent Unexamined Publication Hei 7-57535, a thin film of a perovskite type ferroelectric composition is formed on a substrate, on which a thin film of a perovskite type oxide composition solid solubilizable with the thin film of the perovskite type ferroelectric composition is formed, and a thin film comprising two layers consisting of a thin film of the ferroelectric composition and a thin film of the perovskite type oxide composition is subjected to a heat treatment to solid solubilize and crystallize them, to attain a method of manufacturing a thin ferroelectric film free from the formation of other phases than the perovskite phase and with a uniform crystal grain size.
The conventional capacitor described above in publication (1) involves a problem that the ferroelectric property thereof has a film thickness dependence and no sufficient characteristic can be obtained at a film thickness of 200 nm or less.
Further, as a problem inherent to the thin ferroelectric film capacitor, there is a film fatigue problem in that the dielectric polarizing characteristic is degraded and writing can not be accomplished with repeated reversion of the dielectric polarization many times, as well as an imprinting phenomenon which occurs when it is left polarized in one direction for a long time without conducting rewriting.
Further, the methods described in (2) and (3) involve a problem in that two kinds of thin films of perovskite crystals of quite different compositions have to be formed and this complicates the film-forming apparatus.
An object of the present invention is to provide a ferroelectric memory device which is subject to less film fatigue and imprinting and which has a long life, as well as a method of manufacturing such a device.